This invention relates to plastic and non-metallic article shaping and more particularly to a method for blow molding hollow articles from a heated thermoplastic material with reinforcement ribs formed on the interior surfaces of the article.
In order to save material, plastic bottles which do not require great strength characteristics, such as in half pint bottles for milk and similar drinks, have been formed with thin wall thicknesses. Many of these bottles, however, require reinforcement in the form of externally projecting ribs to compensate for wall thickness. Because these ribs are positioned on the exterior surface of the bottle, they must be arranged to appear as a decorative effect to aesthetically please the consumer. Accordingly, the external reinforcement ribs are generally arranged around the entire circumference of the bottle and sometimes along the entire length of the bottle. Such an arrangement results in two primary disadvantages. First, labelling or decorative space is limited or interfered with by the protruding external ribs. Second, greater amounts of material are required to form the ribs to accomplish the decorative effect. The present invention overcomes these disadvantages by providing a bottle having internal reinforcement ribs which do not interfere with other decoration on the bottle. The internal ribs can be discontinuous so that the bottle can be strengthened only where necessary or desired, and, therefore, which provide for a material savings.
It is known in the prior art to extrude a tubular plastic parison having external reinforcement ribs which are reversed during the blow molding process to provide internal reinforcement to a blown bottle, as described in U.S. Pat. No. 3,114,932 to Donnelly. It is also known in the prior art to injection mold a plastic parison having a localized thickened region which reinforces the heel of a blown bottle, as disclosed in U.S. Pat. No. 3,137,748 to Makowsky. A primary drawback in the method disclosed in U.S. Pat. No. 3,114,932 is the inability to form circumferential grooves in the plastic parison which are destined to become circumferential reinforcement ribs on the container, without providing a variable extrusion die which is costly because of the required machining, tooling and multiplicity of die parts. In still another known prior method, as disclosed in my prior U.S. Pat. No. 3,956,441, it is taught that convex or externally projecting ribs may be molded in a preformed parison. When the preformed parison is subsequently blow molded in a conventional manner, the external ribs in the preformed parison reverse to form wide ribs of a relatively low height on the interior of the bottle. Since the shape of the parison and the locations of the ribs are precisely molded in the preformed parison, they are precisely defined in the finished bottle or other hollow article blown from the preformed parison. The main disadvantages of this method are that it does not permit forming narrow ribs and the height of the ribs are not easily controlled. To increase the height of the ribs, considerably more material is required since the width of the ribs is also increased. In Fuerst et al. U.S. Pat. No. 3,398,428 is disclosed in FIG. 7 a parison having external grooves and adjacent opposing these grooves radially are internal grooves and not ribs as in my invention.